1. Field of the Invention
The present invention relates to an X-ray tube of the rotary anode type and, more particularly, an improvement of the rotating mechanism for supporting the anode target.
2. Description of the Related Art
As well known, in an x-ray tube of the rotary anode type, a disk-like anode target is fixed to a rotary structure which is rotatably supported by a stationary structure and bearings are formed between the stationary and rotary structure. An electron beam is bombarded on the anode target so that X-rays are radiated from the anode target, while exciting electromagnetic coils located outside a vacuum envelope to rotate the rotary structure at high speed. Ball bearings have been used for a long time but it is now expected that bearings of the hydro-dynamic pressure type will become used. In the case of this dynamic pressure type bearings, spiral grooves are formed on the bearing face and liquid metal such as gallium (Ga) and alloy of gallium, indium and tin (Ga-In-Sn) is used as lubricant. Examples in which the dynamic pressure type bearings are used are disclosed in Japanese Patent Publication 60-21463, Japanese Patent Disclosures 60-97536, 60-117531, 61-2914 and 60-287555, for example.
The rotary structure by which the anode target is supported usually includes an outer cylinder made of copper, high in electric conductivity, to serve as a rotor, and a target support welded integral to the rotor is soldered. Rotating magnetic field is applied from a stator located outside the vacuum envelope to the rotor to rotate the rotor at high speed according to the principle of the induction motor. In the case of the X-ray tube in which the ball bearings are used, noises become larger as the temperature of the ball bearings rises higher. This is because the clearance between the bearings is changed and because solid lubricant is fatigued. Various kinds of measure have been proposed to suppress the temperature rise in the ball bearings. Some of them are disclosed in Japanese Patent Disclosures 55-3180, 55-78449 and 2-144836. However, they have not become practically used yet.
The X-ray tube in which the hydro-dynamic pressure type bearings are used is characterized in that rotating noises are hardly created. The X-ray diagnostic instrument in which the X-ray tube is incorporated is often used in intense coldness or at a temperature lower than 0.degree. C. It is therefore preferable that lubricant is made of materials whose melting points are low. Ga alloys are the most suitable for use as lubricant because their vapor pressures are low and their melting points are equal to or near to 10.degree. C.
When one of these Ga alloys is used is lubricant, however, the following drawbacks are caused. Ga alloys are so active as to react with bearing component members. As the result, the clearance between bearings is gradually changed to deteriorate the rotating characteristics of bearings. This limits those materials, of which the bearings are made, to tungsten (W), molybdenum (Mo), tantalum (Ta), niobium (Ni) and alloys of them, which cannot be corroded by Ga alloys. Copper (Cu), tin, iron (Fe), nickel (Ni) and iron alloy such as stainless steel, however, are low in cost and easy to be processed. But they are regarded as being impractical because they can be quite easily corroded. The reaction of bearing component materials with Ga alloys is more remarkable as temperature becomes higher. In order to prevent the bearing component materials from being corroded by Ga alloy, it is now well known that cooling medium is introduced into the bearing component members to forcedly cool the bearing sections. In the case of this X-ray tube, however, a unit for circulating the cooling medium through the system must be added. This makes the X-ray instrument complicated and it is quite undesirable.
U.S. Ser. No. 766,276, filed Sep. 27, 1991, Ono et al, discloses an X-ray tube of the rotary anode type which is provided with a measure for solving the above-mentioned thermal troubles. It is asked, however, that the thermal measure of this X-ray is further improved.